1. Field of the Invention
The present invention relates to a multibeam exposure head with which multiple beams are imaged on a recording material such as a photoreceptor, a silver-halide or non-silver-halide photosensitive material, or a heat mode recording material by exposure, and a multibeam recording method using the multibeam exposure head.
2. Description of the Related Art
Conventionally, in the technical field of printing, lithographic plate making using a presensitized plate (PS plate) has widely been practiced. For instance, lithographic plate making for multicolor printing is performed in the manner described below: A color image is read using a scanner by decomposing into three colors of red (R), green (G), and blue (B). Image signals representing these three colors are converted into color-decomposed halftone-dot-signals for four colors of cyan (C), magenta (M), yellow (Y), and black (Bk). Printing on a photosensitive material called a “lith film” is performed for the respective colors by exposure to light beams modulated on the basis of the color-decomposed halftone-dot-signals for the respective colors to thereby obtain lithographic plates for the corresponding colors. Then, the thus obtained lithographic plates are used to print halftone dot images for the respective colors on PS plates by exposure, thus making printing plates for the four colors of C, M, Y, and Bk for lithographic printing.
In recent years, however, direct plate making and computer to plate (CTP) technology have been put into practical use and have received attention for simplifying a plate making process and reducing the time required for plate making. In the direct plate making and CTP, to make a printing plate for each of the four colors of C, M, Y, and Bk, direct drawing with a light beam such as a laser beam is performed on a PS plate, using the corresponding color-decomposed halftone-dot-signal obtained by a scanner system, and lith films are not used.
Also, in order to increase halftone levels and image quality of printed images, it is desired to increase a recording density. In addition, it is also desired to shorten the plate making time while increasing the recording density. Note that such high-density drawing in a short period of time is desired not only in the technical field of printing but also in various other fields of image recording.
As a technique for shortening the plate making time while increasing the recording density, there are known a multibeam exposure head and a multibeam exposure apparatus disclosed in commonly assigned JP 2002-169113 A, for instance. According to the prior art technique, optical fiber arrays are arranged in two rows so that an upper optical fiber array and a lower optical fiber array can cooperate with each other, and image recording by exposure is performed at a desired resolution while a recording material is moved relative to the optical fiber arrays, with the imaging magnification of an optical system being hardly changed.
According to the multibeam emitting unit disclosed in this prior art document, two optical fiber arrays are sandwiched between fixing members so as to be secured parallel and spaced apart from each other by a predetermined distance, and the positions at one end of beam emitting ports in one of the optical fiber arrays are displaced in a parallel arrangement direction with respect to the positions at the corresponding end of beam emitting ports in the other of the optical fiber arrays.
Also, with this technique, the parallel arrangement direction of the two optical fiber arrays is tilted with respect to a direction (subscanning direction) that is perpendicular to the relative moving direction of a recording material and a beam emitting direction, whereby recording on the recording material can be performed while switching among at least two recording pitches in the subscanning direction of the spots formed by multiple beams emitted from the optical fiber arrays on the recording material.
In particular, by satisfying a specific condition through appropriate settings of the distance between the two optical fiber arrays arranged parallel to each other and the shift amount between these optical fiber arrays in the parallel arrangement direction, an exposure condition having a target recording pitch can be obtained at the time of exposure on the recording material merely by rotating the two optical fiber arrays by a predetermined angle using an exposure head tilt angle changing device. The prior art head and apparatus are also advantageous in that multiple high-resolution exposure conditions can be set using a fine magnification adjustment lens.
The multibeam exposure head described above is capable of switching between a first exposure condition and a second exposure condition using the exposure head title angle changing device described above. Here, under the first exposure condition, each of multiple beams emitted from the upper one of the two optical fiber arrays arranged parallel to each other and each of multiple beams emitted from the lower one of the two optical fiber arrays are alternately arranged at equal intervals in the subscanning direction that is perpendicular to the beam emitting direction and a main scanning direction on the recording material. On the other hand, under the second exposure condition, the two optical fiber arrays are tilted about a rotation axis extending parallel to the multiple beam emitting direction described above so that each of the multiple beams emitted from the upper optical fiber array and each of the multiple beams emitted from the lower optical fiber array are alternately arranged in the subscanning direction at equal intervals that are different from the equal intervals under the first exposure condition.
With this multibeam exposure head, however, if the upper optical fiber array and the lower optical fiber array constituting the dual-stage construction are not arranged parallel to each other with high accuracy, variations occur in the recording pitch at which the recording material is exposed to the multiple beams at the time of scan-recording. As a result, image unevenness is generated as a defect.
That is, in order to perform recording of high quality images under both of the first exposure condition and the second exposure condition, it is required that each of the multiple beams emitted from the upper optical fiber array and each of the multiple beams emitted from the lower optical fiber array are arranged parallel to each other with high accuracy. However, it is extremely difficult to arrange them parallel to each other with high accuracy.
If the upper optical fiber array and the lower optical fiber array are sandwiched between the fixing members to be secured parallel to each other for the arrangement with higher parallel degree, a problem of cost increase may arise.